The cellular slime mold, Dictyostelium discoideum, is an ideal organism for research on the control of cell proliferation. In its life cycle the vegetative amoebae first proliferate and then become quiescent as they differentiate in response to starvation. Genetic and molecular analysis of the transition revealed three classes of genes: "vital" genes expressed at all stages, proliferation genes selectively expressed only during cell proliferation and differentiation genes expressed only during development. The putative proliferation genes could include a novel set of genes essential for DNA synthesis, mitosis, cytokinesis or other aspects of cell proliferation. In spite of considerable effort in other systems relatively few cell division cycle genes have yet been found. The major objectives in the research include using cDNA probes of the three classes comparatively to determine specific mechanisms of proliferation gene expression. Filter hybridization methods will be used to identify the most likely candidates by their regulation and conservation in evolution. Some of the genes have homologies to mammalian sequences. Additional homologies to known cdc genes will be sought by hybridization studies and sequence analysis. The turn off of the genes is under genetic control. The mutations may be in regulatory factors which control cdc gene expression. Thermosensitive conditional mutants will be isolated by a novel selection protocol. A current hypothesis that cAMP represses these gene's expression will be assessed by RNA blot analysis of cAMP chemosensory mutants. The final result will be an identification of known and novel proliferation genes and an identification of genes and physiological events which control their expression. Although mammalian cells share some of the same genes studied here, they lack the experimental advantages of this system. Therefore, knowledge about the functions and regulation of these genes may aid in understanding developmental diseases and neoplastic growth.